Eddy current testing systems have heretofore been provided for locating and measuring the severity of surface and near-surface flaws in both magnetic and non-magnetic conductive materials. Such systems have used test coil assemblies in which an output signal is developed corresponding to the difference between signals developed by two windings which are inductive and coupled to two parts or to separate portions of one part. In the testing of elongated parts such as rods, tubes or wires, windings have been provided around longitudinally spaced portions of the part and, as the part is moved through the windings, cracks or other inhomogeneities may be detected from the difference in signals produced by the two windings.
Such eddy current systems have been used extensively and with very satisfactory results in most applications and the possibility of improving the resolution and sensitivity of the systems has not been generally recognized.